1. Field of the Invention
This invention relates to prosthetic appliances for humans and other animals, and in particular it relates to a prosthetic device to occupy the chest cavity left vacant after excision of a lung. The prosthesis of this invention performs the separating and supporting functions performed by the lung, without interfering with the performance of the heart or any other organs and without impairing the breathing function of the remaining lung. This invention also relates to a method for making and using such a prosthesis.
2. Description of Related Art
The removal of one lung (i.e. pneumonectomy) is a surgical procedure not uncommonly performed for treatment of cancer, trauma, or infection of the lung. Prevention of overdistension of the remaining lung after pneumonectomy has been a concern to thoracic surgeons for many years. They anticipated that overdistension would cause distortions of remaining structures inside the thorax leading to abnormal heart and lung function. Many surgical procedures were designed to prevent overexpansion. In the 1940's investigators placed hollow lucite balls in the vacant chest cavities of dogs and humans to prevent overdistension of the remaining lung and to maintain the mediastinum in the midline following pneumonectomy. This work was described in Johnson et al., Journal of Thoracic Surgery 18:164 (1949).
Other substances and devices have been used to replace an excised lung and to correct the problems associated therewith, including gelatin foam, plastic sponge (Ivalon) and a polythene bag filled with fiberglass. Since the early 1960's these procedures have generally been abandoned as unnecessary.
More recently, it was noted that in several very young patients respiratory difficulties occurred after pneumonectomy due to tracheal kinking and deformity from marked mediastinal shift following pneumonectomy. A degree of success was reported by physicians who implanted silicone testicular and. breast prostheses in the thoracic cavities of infants and young children to avoid and cure such difficulties, as was reported in Powell et al., Journal of Pediatric Surgery 14:231 (1979).
Success in correcting tracheal shift and overdistension of the remaining lung following pneumonectomy was also reported after implantation of silicone implants (similar to breast implants) in a 25 year old man in 1975, as was reported in Wasserman et al., Chest 75:78 (1979).
Use of an expandable prosthesis to remedy problems associated with a removed lung was reported in Rasch et al., Annals of Thoracic Surgery 15:127 (1990). The reference indicated that an inflatable tissue expander with a subcutaneous injection port was implanted in a 5 month old infant. A 125-mL Surgitek inflatable tissue expander was inserted into the right pleural space and inflated with 60-mL of saline solution, which effectively returned the heart and mediastinal structures to near their natural positions. Five months after the operation the patient's respiratory difficulty returned and was relieved by injecting an additional 30-mL of saline solution into the prosthesis.
Numerous types of tissue expanders have been used by medical practitioners for non-thoracic purposes, such as those disclosed in, for example, U.S. Pat. Nos. 4,095,295, 4,643,733, 4,800,901, 4,685,446, and 4,969,899. These are all designed for use in muscular and cutaneous tissue and are filled with a fluid or gelatinous material. Ordinary tissue expanders are not well suited for use as lung prostheses, however, because (a) they do not conform to the shape of the lung, so normal anatomy is not restored, (b) they generally contain saline, which is heavy and which does not restore normal compliance around the heart, and (c) they are not expandable to the extent required to permit implantation in a child and subsequent expansion to accommodate normal growth. A normal lung comprises three major exterior surfaces. The "rib cage surface" is convex and is adjacent to and conforms to the configuration of the inner surface of the rib cage. The lower "diaphragmatic surface" is concave and is adjacent to and conforms to the configuration of the thoracic surface of the corresponding hemi-diaphragm. The "cardiac and mediastinal surface" is generally concave and irregularly shaped to conform to the contours of the heart and large mediastinal vessels such as the aorta and the pulmonary arteries. The contour of this surface varies significantly from patient to patient because there are considerable variations in the normal shape and size of the heart. The right lung normally comprises 3 lobes (upper, middle and lower) and the left lung normally comprises 2 lobes (upper and lower) separated by fissures. In response to injury or disease, an entire lung or one or more lobes of a lung may be resected.
Other space-filling artificial organs have constructed, for example the artificial bladder disclosed in U.S. Pat. No. 4,044,401 is intended to occupy a space normally occupied by a bladder. Such a device is preferably rigid and made of hard plastic, making it unsuitable for use in a dynamic and structurally constrained location, such as the thoracic cavity. Furthermore, such devices are not designed to be filled with inert gasses or other materials, allowing proper fit and expandability following growth of the patient
It has also been long known that the work tolerance of a human is significantly lowered following pneumonectomy, and it was assumed that the reduced tolerance was due to lowered respiratory capacity. The inventors and others have recently determined that the remaining lung is often adequate to supply the body's oxygen mediastinal shift and overdistension and displacement of the remaining organs after pneumonectomy is provided. The lung prosthesis according to this invention is hollow (or fluid-filled) and lightweight, yet it effectively and compliantly supports the heart and mediastinal structures and inhibits their movement from normal positions, preventing. adhesion of the heart to other structures to avoid fibrous encasement of the heart. The prosthesis may be custom molded to fit the chest cavity into which it is intended to be placed and provided with a subcutaneous septal port which allows fluid to be injected into and withdrawn from the implanted prosthesis without additional surgery. An elastic material may be chosen to construct the prosthesis to permit volume changes as the patient grows.
A method is provided for custom molding a prosthesis according to this invention to match the size and shape of the chest cavity of each patient, although it may be feasible to provide a plurality of prostheses across a range of sizes, either individually or in a kit, from which a physician could select a suitable prosthesis. The prosthesis may be designed to allow for uniform expansion of the prosthesis to accommodate growth of a young patient while maintaining its anatomically correct shape over a range of volumes.
The presently preferred custom molding method of making a balloon prosthesis according to this invention comprises obtaining a computer model of the chest cavity the prosthesis is intended to occupy using non-invasive imaging techniques, for example, magnetic resonance imaging or computer tomographic (CT) scanning. A three dimensional physical model may be made based on the computer model, from which a mold may be constructed to form the balloon. The computer model may be generated at the hospital or clinic where the patient is examined and transferred to the balloon fabricator, or it may be generated by the fabricator.
The invention also includes a method for preventing mediastinal shift and overdistension and displacement of organs following pneumonectomy, comprising providing a prosthetic balloon as described herein, implanting it in a patient following needs, and that the reduced ability to work is due to other physiological changes that occur after pneumonectomy. The heart is ordinarily held in place by the cardiac fossa, a compliant cavity formed largely by the shape of the lungs around the heart. When one lung is removed, it has been observed that the heart tends to be pushed against the rigid rib cage and to become attached thereto by relatively non-compliant fibrous tissue. These anatomical changes result in the heart muscle being surrounded by much less compliant structures than is normal, reducing the heart's ability to expand freely and thereby impairing cardiovascular performance.
It is therefore desirable to provide a method of preventing mediastinal shift and overdistension and displacement of the visceral organs following pneumonectomy, and to provide a prosthesis to occupy the space in the chest cavity vacated after pneumonectomy to maintain a compliant cardiac fossa, to prevent the heart from being displaced from its normal position, to inhibit overdistension of the remaining lung, and to maintain the mediastinum near the midline. It is further desirable to provide as light a prosthesis as possible to reduce the work required to support it, while providing a safe and reliable device for long term implantation. Also, it is desirable that a lung prosthesis be adapted to permit adjustment to its volume by injection and withdrawal of fluid (liquid or gas) without additional surgery, and that the prosthesis be expandable to permit its use in a growing patient by periodically injecting additional fluid to increase the volume of the prosthesis. Finally, methods by which such a prosthetic device may be constructed and used are desired. The present invention provides a solution to the long-felt need for a functional prosthesis that overcomes many of the limitations of prior developmental attempts.